Even though there are significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulator, there are still limitations and difficulties when employed to provide high quality images display. Specifically, when the display images are digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. Referring to FIG. 1A for a digital video system 1 disclosed in a relevant U.S. Pat. No. 5,214,420 that includes a display screen 2. A light source 10 is used to generate light energy for ultimate illumination of display screen 2. Light 9 generated is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13 and 14 form a beam columnator to operative to columnate light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. The SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 32, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30 that shown in FIG. 1B. When element 17 is in one position, a portion of the light from path 7 is redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, light is not redirected toward display screen 2 and hence pixel 3 would be dark.
The on-and-off states of micromirror control scheme as that implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display system imposes a limitation on the quality of the display. Specifically, when applying conventional configuration of control circuit has a limitation that the gray scale of conventional system (PWM between ON and OFF states) is limited by the LSB (least significant bit, or the least pulse width). Due to the On-Off states implemented in the conventional systems, there is no way to provide shorter pulse width than LSB. The least brightness, which determines gray scale, is the light reflected during the least pulse width. The limited gray scales lead to degradations of image display.
Specifically, in FIG. 1C an exemplary circuit diagram of a prior art control circuit for a micromirror according to U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where * designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors, M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads presented to memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a wordline. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The dual states switching as illustrated by the control circuit controls the micromirrors to position either at an ON of an OFF angular orientation as that shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally control image system is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror is controlled at an ON position is in turned controlled by a multiple bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” when control by a four-bit word. As that shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8 that in turn define the relative brightness for each of the four bits where 1 is for the least significant bit and 8 is for the most significant bit. According to the control mechanism as shown, the minimum controllable differences between gray scales for showing different brightness is a brightness represented by a “least significant bit” that maintaining the micromirror at an ON position. For practical implementation, the control signals with different binary bits are inputted to the control circuits as shown in FIG. 1E. There is generally a small time gap between control words of different bit lengths as shown.
When adjacent image pixels are shown with great degree of different gray scales due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to image degradations. The image degradations are specially pronounced in bright areas of display when there are “bigger gaps” of gray scales between adjacent image pixels. It was observed in an image of a female model that there were artifacts shown on the forehead, the sides of the nose and the upper arm. The artifacts are generated due to a technical limitation that the digital controlled display does not provide sufficient gray scales. At the bright spots of display, e.g., the forehead, the sides of the nose and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities.
As the micromirrors are controlled to have a fully on and fully off position, the light intensity is determined by the length of time the micromirror is at the fully on position. In order to increase the number of gray scales of display, the speed of the micromirror must be increased such that the digital control signals can be increased to a higher number of bits. However, when the speed of the micromirrors is increased, a strong hinge is necessary for the micromirror to sustain a required number of operational cycles for a designated lifetime of operation, In order to drive the micromirrors supported on a further strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The micromirrors manufacture by applying the CMOS technologies probably would not be suitable for operation at such higher range of voltages and therefore the DMOS micromirror devices may be required. In order to achieve higher degree of gray scale control, a more complicate manufacturing process and larger device areas are necessary when DMOS micromirror is implemented. Conventional modes of micromirror control are therefore facing a technical challenge that the gray scale accuracy has to be sacrificed for the benefits of smaller and more cost effective micromirror display due to the operational voltage limitations.
There are many patents related to spatial light modulation that includes U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541,4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions have not addressed and provided direct resolutions for a person of ordinary skill in the art to overcome the above-discussed limitations and difficulties.
Therefore, a need still exists in the art of image display systems applying digital control of a micromirror array as a spatial light modulator to provide new and improved systems such that the above discussed difficulties can be resolved.